Piston ball end



G. A. WAHLMARK 2,947,182 l Aug. 2, 1960 PISTON BALL END 2 Sheets-Sheet 1 Filed May 7. 195,8

www mw INVENTOR. afzfzafd/a/zmr/Q BY @0g M M+ M' y'tetes at y PISTON BALL END Gunnar A. Wahlmark, 211 S. Rockford Ave., Rockford, Ill.

Filed May 7, 195s, ser. No. 733,716

s cxaims.- (Cl. 'r4-60) This invention relates to improvements in piston members. More particularly, the invention relates to' improvements in the piston members employed in swash plate or wobble `plate hydraulic devices.

`This application is a continuation-impart of my copending vapplication entitled Piston, Serial No. 583,797, filed May 9, 1956. t

An axial swash plate or wobble plate hydraulic device such as a pump or a hydraulic power transmission ernploys pistons which are reciprocated by means of a circular plate mounted obliquely on a rotatable shaft. If the obliquely mounted plate is connected to the shaft by means of a'unive'rsal joint and the pistons are rotated with the plate, then the device will be hereinafter referred to as swash plate device. If the plate isv obliquelymounted to oscillate or wobble as the shaft is rotatedand the Vpistons are not rotated with -the plate, then the device willbe called a wobble plate. device. In the, past all-suchdevices have been generically referred to as swash plate devices, .but in recent practice the above Aprecise terminology has been adopted` Yto differentiate between the two types. f c ,l ,v

In a swashA plate device where the pistons are rotated with the plate, serious diiiculties have been encountered with respectrtopiston and` cylinder-wall wearV caused by centrifugal force.` Furthermore, rotation of thetpisto'ns angular-ity between theaxes of theqpistons and the axis of the shaft,and these angularity changes necessitate employment of universal connections between vthe piston rods'and the pistonheads. Since tlie Jchanges in angularity are slight and the loads against the pistonV heads are quite large, serious difficulties in lubrication are encountered in such universal connections.

In all swash plate or wobble plate devices utilizing conventional pistons the piston heads must be of substantial axial length, thus introducing difficulties in construction and operation due to close tolerances necessitated by high hydraulic working pressures. Close tolerance cylinder and piston combinations are dicult and expensive to. manufacture and are subject to excessivewear or seizing when abnormally highhydraulic fluidtemperatures or, sudden changes in temperature are encountered. Where a universal connection is utilized between the pistonhead and the piston rod, the pistoncannot be completely pressure balanced between the-swash plate. and the oppo'site end of the piston head, since the joint must necessarily, be smaller than the ,outside piston diameter.

An important object of the invention is to provide improved pistonsv for use in swash plate orwobble plate hydraulic'devices; f Y A further object is to provide an improved piston device capable of `substantial change in angularity without leakage or excessive wear. Y v Y Y Anadditional object of the inventionis to provide a piston de vice `Whichis substantially lighter than arco'nventional piston of similarcapacity.

A further object of the invention is to provide in a hydraulic device a piston which is capable of universal pivoting while maintaining complete hydraulic balance.

A still further object of the invention is to provide an improved universal ball end fo'r connecting a piston rod to an actuating member. Y

Another object of the invention is to provide an improved universal ball end which permits extremely close placement of pistons in a swash plate hydraulic device in order that the hydraulic port velocity can be substantially reduced.

Other objects, features and advantages will be apparent from the following detailed description taken in conjunction with the accompanying drawing, in which:

Figure l is a sectional view of an axial swash plate pump embodying an improved piston according to the present invention; and l Y Figure 2 is an enlarged fragmentary view of the universal ball end portion of the piston shown in Figure l.

In Figure 1 is illustrated an axial swash plate hydraulic device in the fo'rm of a variable displacement pump 2G. The pump 20 includes a housing or casing-21 containing swash plate hydraulic pumping mechanism 22.

The housing 21 includes an open ended casing 23 having its open end closed by a cover plater24. The cover plate 24 may be xedly secured to the casing 23 by means of a plurality of bolts 25. kThe casing 23 is provided with an attachment ilange 26 containing attachment holes 27 whereby the housing 21 may be secured to a Vdriving'structure such as an air-craft engine (not shown). Y

Vwith the plate introduces slight changes inthe ,universal t The cover plate 24 is provided with an outlet or pressure aperture 28 which includes an outlet or pressure port 29, and an inlet or suction aperture 30 which includes an inlet or suction port 31. The inlet port may be connected to an engine oil or hydraulic Huid source (not shown) and the outlet aperture 28 may be connected to the pressure side of ahydraulic system in an aircraft or guided missile, for example (not shown). Y

The pumping mechanism 22 includes anY obliquely mounted, axially oscillatable periphery plate assembly 32 and a cylinder block 33. The plate assembly 32 is of the typejreferred to as a .swash plate and includes an outer non-rotating tiltable displacement control ring .34, within which a piston socket ring 35 is rotatably secured by means o-an anti-friction thrust-bearing 36. The piston socket ring 35 is universally tiltably connected to an inner ring 37 by means of a universal joint 3SY which is preferablyof the constant velocity type as shown. The inner ring 37 is connected by means of splines 39 toY a drive sleeve 40. Y t

The cylinder block 33 contains a-plurality of pumping cylinders or chambers 41 (one shown) whichv are disposed in radially spaced relation The particular pump 20Lis provided with-nine pumping cylinders. .The cylinders 41 are open at their ends facing the Vswash plate assembly 32 andare provided with cylinder feed ports 42 at their opposite ends. The cylinder block 33 Vhas a radial cylin= der feed port face 43 disposed in rotating slidableabutmentnagainst an inner radial face 44 of the cover plate 24. The cylinder block 33lis rotatably. supported with respect to the cover plate 24 by means ofan Vanti-friction bearing 45 which has its inner race securedto a clamping lug 46 .which'in turnis connectedto the cover' plate 24 by means of a bolt 46a. Internal splines 47 formed on the cylinder block 33 are engaged with external splines 47a formed on the end of the drive sleeve 40 whereby the cylinder block is adapted-for being rotated by the drive sleeve. t 1

, For rotating the drive sleeve 40, a drive shaft 48 is inserted through an axial aperture 49 formedin the casing 23. An externally splined inner end 50 of the drive shaft is engaged with internal splines 51 formed in the drive sleeve. The outer end of the drive shaft 48 is provided with external splines 52 which are adapted for engagement with an auxiliary drive mechanism of an air-craft engine, for example (not shown). In order to prevent leakage from inside the casing 23 through the aperture 49, rotating seal means 53 are disposed between the portion of the casing 23 deiining the aperture 49 and the end of the drive sleeve 40. j

According to the present invention, a plurality of pistons 54 are operatively disposed in the pumping cylinders 41. Each of the pistons 54 includes a universal ball end portion 55 at one end, a head portion 5'6 at the other end, and a piston rod portion 57 therebetween. The ball ends 55 are universally disposed in respective mating universal ball sockets 58 provided in radially spaced relation about the piston socket ring 35 of the swash plate assembly 32. The ball sockets 58 are formed in hardened socket liners '59 which are ixedly secured in the ring 35. The head portions of the pistons 54 are disposed in pumping relation within the respective pump cylinders 41.

From the description thus far, it will be seen that rotation of the drive shaft 48 drives the sleeve 40 and causes concurrent rotation of the cylinder block 33 and the piston socket ring 35. As these portions of the pumping mechanism Y22 Yare rotated, the pistons 54 are reciprocated in the cylinders 41 due to the oblique mounting of the plane of the swash plate assembly 32 with respect to the axis of rotation which causes any given point on the peripheral portion of socket ring 35 to oscillate axially. The stroke of the pistons 54, and consequently the displacement of the pump, depends upon the angle between the swash plate plane and the axis of rotation. If the plane of the swash plate were perpendicular to the axis of rotation, the pistons 54 would not reciprocate and the pump displacement would be zero. As the plane of the swash plate assembly is canted more and more with respect to the axis of rotation, the pump displacement increases. A pump such as that illustrated in Figure l is designed to operate at three thousand pounds persquare inch (3000 p.s..) maximum pressure `and at high rotational speeds in the order of 25,000 revolutions per minute or higher.

As the pumping mechanism 22 is rotated through a complete rotation, the pistons S4 are universally pivoted slightly with respect to the axes of the cylinders 41, due to the fact that the ball ends 55 are slightly closer to the axis of rotation when the pistons are at the opposite ends of their strokes, than when the pistons are in between the stroke ends. The ball ends are at their maximum distances from the axis of rotation at the points 90 removed from the lpositions of top dead center and bottom dead center. Ordinarily the construction and arrangement of swash plate pumps are such that the total universal pivoting of the pistons is not more than about two degrees at maximum swash plate angle. v

In order to Vary the displacement of the pump, any suitable means may be employed (not shown). Ordinarily such means are operatively connected to the tiltable displacement control ring 34.

Utilization of the constant velocity universal joint 38 insures that the piston socket ring rotates at a constant velocity with respect to the drive shaft 48, regardless of the angle between the plane of the swash plate assembly and the drive shaft. Use of an ordinary or Cardan type of universal joint is undesirable due to the periodic acceleration and deceleration of the driven member which occur twice during each complete yrotation of the driving member. v

In addition to the other advantages, pistons according to the present invention provide for universal pivoting of the piston without necessitating incorporation of a separate universal joint betweenthe piston head and the shaft. Conventional piston head and piston rod combinations utilizing universal connections in the piston heads 4 are illustrated in Williams and Janney Patent No. 1,062,- 071 and Thoma Patent No. 2,155,455.

The outlet or pressure port 29 and the inlet port 31 in the cover plate 24 may be of arcuate or kidney configuration (not shown) such as illustrated in Williams and Janney Patent No. 1,062,071, referred to above. In this patent the kidney inlet and outlet ports are illustrated in Figures 7 and 8 of the drawings and bear the reference numerals 27 and 28, respectively. In the present construction the outlet and inlet ports 29 and 31 are arranged so that the feed ports 42 each communicate with either the inlet port or the outlet, or are blocked by the lands in between provided by the material of the cover plate between-the ports. The ports are arranged so that the respective feed ports 42 ordinarily communicate with either the outlet .port 29 or the inlet port 31. A similar port arrangement is shown in the Williams and Janney patent.

The pressure in the outlet port 29 acts against the exposed land area of the cylinder block 33 between adjacent cylinders (as shown opposite the inlet port 31 in Fig. l), tending to force the cylinder block away from the cover plate 24. In addition, a limited amount of pressure uid tends to seep between the adjacent faces of the cover plate 24 and the cylinder block 33 in the region of the feed ports, and this leakage pressure also tends to force the cylinder block away from the cover plate. This separating force must be counteracted by the pistons through the pressure acting in the cylinders ahead of the piston heads 56. In each cylinder the pressure acts against an area equal to the cross sectional area of the cylinder less the area of the outlet port, providing a force tending to hold the cylinder block against the cover plate. The areas `are arranged so that the force tending to seat the cylinder block 33 is slightly larger, ve percent for example, than the force tending to unseat the cylinder block.

In a high pressure, high speed hydraulic device such as that of the present invention it is important that the areas of the feed ports 42 be as large as possible in order to reduce the velocity of fluid passing through the ports. It will be apparent that if the port velocity is high, large pressure losses are encountered which reduce the eticiency of the hydraulic device and cause undesirable overheating. In most instances, the permissible speed of the hydraulic device is limited by the port velocity, and, consequently, by the areas of the cylinder ports.

In order to provide complete hydraulic balancing of the pistons, the ball ends 55 (Fig. 2) are provided with a primary ball segment 78 which has a maximum circumference 79. An axial passage is provided in the piston rod portion 57 and extends through the piston head portion 56 and the ball end portion 55, so that the pressure communicated to the exposed area of the piston head is also communicated to the opposing area of the primary ball segment 78. The projected area of the primary ball segment 78 is formed sutciently large that the pressure force acting against the ball end 78 is substantially equal to the pressure force acting against the piston head, so that the piston is completely hydraulically balanced on its pressure stroke.

The effective pressure in the socket 58 acting against the primary ball end 78 4is less than the full pressure in the cylinder ahead of the piston head inasmuch as this pressure is communicated between the adjacent surfaces of the socket 58 and the ball end 78. At the opening to the axial passage 80, the pressure is substantially equal to working pressure, but at the outer edges of the primary ball '78, the `pressure -is reduced almost to case ing pressure, thus resulting in a reduced over-all effective pressure. Consequently, the projected area of the primary ball segment 78 is at least equal to and is preferably somewhat larger than the cross sectional area of the piston head and the cylinder. It has been found that if .the diameter of the maximum circumference 79 of the primary ball segment is approximately ten to fifteen percent larger than the diameter of the cylinder, complete hydraulic balancing will be achieved. i

The universal ball portion 5S of thepiston 54 is improved over the conventional full ball design as illustrated yin the two patents referred to above. As distinguished from a full ballV end having the same diameter as the piston cylinder, the primary ball segment 78has a spherical diameter larger than the cylinder diameter, but the total surface of the ball is less than a hemisphere. Thus, a relatively ilatter Vsurface is provided with the same effective` pressure area for the purpose of maintaining hydraulic balance.

In order to maintain the ball end 55 seated within the socket 58 on the suction stroke of the piston, a smaller diameter secondary ball segment 78a lis provided and an annular relief groove 78b is formed between the two ball segments. The secondary ball has a spherical diameter considerably smaller than the diameter of the cylinder. This secondary ball is seated against a secondary socket member 59a which is lixedly secured over the ball socket liner 59 and has an internal segmental spherical surface 58a which matches that of the secondary ball 78a. The segmental balls 78 and 78a have a common center so that the ball end 55 is universally pivotally secured within the socket 58 and is retained therein during both directions of movement of the piston.

In order that the land areas between adjacent cylinders be kept as small as possible so that smaller compensating areas, and consequently larger ports can be utilized, it is highly desirable to place the pistons and cylinders as close together as possible. This, in turn, is limited by strength requirements for the cylinder block 33 and the swash plate assembly 32.

An important limiting factor in close placement of pistons is the requirement for suticient material between adjacent -ball ends in the swash plate. By utilizing the improved ball ends 55 of the present invention in place of conventional full ball ends, it is possible to place the ball ends, and consequently the pistons, considerably closer together, since the provision of the reduced diameter ball portion 78a permits the inclusion of considerably more stock between the ball ends toward the shanks of the pistons. Since the enlarged ball segment 78 is bottomed against the solid piston socket ring 35, it is possible to place the ball ends so close that the sockets 59 touch vone another. In other words, practically no stock is required between the edges of adjacent ball sections 78. This, in turn, permits extremely close placement of the pistons, reducing the exposed land areas between piston ports and substantially reducing the port velocity.

Several additional advantages are aiorded by the ball end construction 55. This type of ball end is considerably reduced in weight from the conventional type so that inertia and centrifugal forces are smaller resulting in reduced wear. Since the ball end of the present invention is somewhat smaller than a comparable size conventional full ball end, the space requirements are reduced. The improved ball end is considerably easier to lubricate since the area of contact is' reduced and the two ball portions are split to provide a lubricant reservoir in the groove 78b and within the uniilled portion of the socket 58. As an additional advantage the improved ball end construction provides partial pressure balance on the suction stroke of the piston. Slight leakage is provided around the primary ball 78 on the pressure stroke so that a residual pressure will remain in the lubricant reservoir. Since the ball segment 7S is considerably larger in diameter than the ball segment 78a, a substantial pressure area dilerential is provided. The pressure in the lubricant reservoir acts against this differential area to reduce the force acting on the ball portion 78a on the suction stroke.-

Variations and modiiications may be effected Without departing from the scope of the novel concepts of the present invention.

I'claim:

1. In a piston having a head portion, a segmental spherical surface formed on the end of said piston opposite the head portion with a spherical diameter larger than the diameter of the head portion and with a maximum diameter at least equal to the diameter of said head portion, anda second segmental spherical surface formed on said piston adjacent said rst mentioned segmental spherical surface but diametrically opposed thereto, said second segmental spherical surface having a spherical diameter smaller than the diameter of said head portion.

2.7ln a piston including a rod portion and a head portion formed at one end of said rod portion, a segmental spherical ball portion formed on the end of said rod 'portion opposite to said head portion, said ball portion having a spherical diameter greater than the diameter of said head portion and having a maximum diameter at least equal to the diameter of said head portion, and a second segmental spherical ball portion formed about said rod portion adjacent said rst named ball portion in diametrically opposite relation thereto, said second ball portion having a spherical diameter smaller than the diameter of said head portion with the spherical centers of said two ball portions coinciding.

3. A piston adapted for reciprocation in a cylinder and having a rod portion and a head portion formed at one end of said rod portion, the improvement comprising a segmental spherical ball formed on the end of said rod portion opposite to said head portion, said ball being less than hemispherical and having a maximum circumference at least equal to the circular circumference of the cylinder, an annular segmental spherical ball formed on said rod portion adjacent said rst named ball in diametrically opposite relation thereto, the maximum diameter of said segmental spherical portion being less than the maximum diameter of said rst named ball and the spherical centers of said balls coinciding, and means defining an axial passage through vsaid rod portion and through said head portion and said balls to provide equalized pressure against the head portion and the rst named ball.

4. An improved piston adapted for reciprocation in a cylinder and comprising a rod portion, a head portion on one end of said rod portion, a segmental spherical ball portion formed on the ,end of said rod portion opposite to said head portion, said ball portion being less than hemispherical and having a maximum circumference at least equal to the circular circumferenceof the cylinder, and a second segmental spherical ball portion formed about said rod portion adjacent said first named Y ball portion in diametrically opposite relation thereto,

said second ball portion having a spherical diameter smaller than the diameter of said head portion with the spherical centers of said two ball portions coinciding.

5. Ln a piston having a rod land a lhead secured there-to, a universal ball member formed on the other end of said rod comprising a segmental spherical ball portion less than Vhemispherical and having a maximum circumference larger than the maximum circumference of the head, a second segmental spherical ball portionl formed about said rod adjacent said first named ball portion in diametrically opposite relation thereto, said second ball portion having a spherical diameter smaller than the diameter of said head portion with the spherical centers of said two ball portions coinciding, said ball member having an annular groove separating said ball portions and providing a lubricant reservoir.

6. A universal ball member for al piston according to claim 5 wherein the maximum circumference of said rst named segmental spherical ball portion is between ten and fteen percent larger than the maximum circumference of the head.

7. In a piston, la rod, a head secured at one end of said rod, a segmental spherical ball portion formed at v7 the other end of said rod and having a spherical diameter greater than the diameter of said head, a second segvmental 4spherical ball portion formed about said rod adjacent said iirst named ball portion in diametrically opposite relation thereto and With its spherical center coinciding with the spherical center of said rst named ball portion, said second ball portion having a spherical diameter smaller than the spherical diameter of said head portion and having a maximum diameter smaller than the maximum diameter of said first named ball portion.

8. A piston rod having a universal ball member comprising first and second segmental spherical ball portions formed in diametrically opposite relation with the spherical centers of the two ball portions coinciding, said rst Y References Cited in the le of this patent UNITED STATES PATENTS 102,703 Pease May 3, 1870 1,062,071 Williams et al. May 20, 1913 1,710,567 Carey Apr. 23, 1929 l FOREIGN PATENTS 24,436 Great Britain Oct. 2S, 1912 

